Hydrogen sulphide sampling method

ABSTRACT

A method for sampling a sulphur-containing solid product including supplying a gas flow comprising hydrogen sulphide, bringing the gas flow into contact with a solid reagent and reacting the solid reagent with the hydrogen sulphide contained in the gas flow, the reaction fixing the sulphur of the hydrogen sulphide by forming a sulphur-containing solid product which is different in colour from the solid reagent, and recovering the sulphur-containing solid product. The invention also relates to a device suitable for the implementation of this method.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/988,771, filed Oct. 20, 2010, which claims priority as a 371 PatentApplication of International Patent Application No. PCT/IB09/05289,filed Apr. 20, 2009, which claims priority to French Patent ApplicationNo. 08 02 241, filed Apr. 22, 2008, each of which applications isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method for sampling hydrogen sulphide(H₂S), as well as a device suitable for the implementation of thismethod.

TECHNICAL BACKGROUND

The management of hydrogen sulphide (H₂S) plays a major part in thefield of gas and oil production. In fact, hydrogen sulphide, which canbe present in a molar concentration that can vary greatly depending onthe deposits (from a few ppm to several tens of %), is a gas which isnot only extremely toxic (fatal at a low concentration) but alsocorrosive in the presence of water. It is therefore important to treatit, as well as to adopt installations adapted to its presence.

The concentration of hydrogen sulphide can increase dramatically, oreven appear, during production, requiring complex and costly adaptationsof the production methods. Such an increase or appearance can haveseveral sources, some natural and others artificial.

Thus, examples of possible sources of hydrogen sulphide are:

-   -   thermal reduction of sulphates by the hydrocarbons at a high        temperature;    -   bacterial reduction of sulphates;    -   in the case of injection of hydrochloric acid into the rock        during production, reaction of the former with pyrite;    -   contamination of the considered reservoir of hydrocarbons by a        second reservoir of hydrocarbons having a higher H₂S content.

It is important to identify the sources of the hydrogen sulphide foreach deposit, in order:

-   -   to reduce when possible the hydrogen sulphide content (for        example by means of a bactericidal treatment or by interrupting        the injection of hydrochloric acid depending on the case); or    -   to predict the evolution over time of the H₂S content, in order        to dimension the installations accordingly, preferably to within        one-tenth of percent of hydrogen sulphide.

This identification of the sources of hydrogen sulphide is difficult; inthe first place it is based on the isotopic measurement of the sulphurpresent in the hydrogen sulphide. In fact, depending on the chemical orbiochemical processes at the origin of the formation of the hydrogensulphide, the isotopic fractionation (i.e. the proportion of heavyisotope ³⁴S involved in the various conversion processes) varies. Ameasurement of the ratio of molar concentration of the heavy isotope ³⁴Sto the majority isotope ³²S (isotopic ratio) therefore provides directinformation on the origin of the hydrogen sulphide, which is a valuableitem of information for production strategies.

In order to carry out the measurement of the isotopic ratio of thesulphur, it is usual to oxidize the hydrogen sulphide beforehand. Theoxidation-reduction reaction is brought about by bubbling the gascontaining the hydrogen sulphide through a solution containing cadmiumacetate. The cadmium acetate reacts with the hydrogen sulphide to formcadmium sulphide. The actual isotopic ratio measurement is carried outon the sulphur present in the cadmium sulphide thus obtained.

The measurement of the isotopic ratio of the sulphur is carried out bymass spectrometry. This measurement therefore requires heavy laboratoryequipment; it cannot be carried out directly on the site.

Two pre-analysis sampling methods are currently used. These involve:

-   -   taking a sample of gas containing hydrogen sulphide on site in a        pressurized bottle, and sending the pressurized bottle to the        laboratory where the isotopic analysis will take place (the        intermediate reaction with the cadmium acetate solution        therefore takes place in the laboratory); or    -   taking a sample of gas containing hydrogen sulphide, carrying        out the reaction of hydrogen sulphide with the aqueous solution        of cadmium acetate on site, and sending the solution obtained to        the laboratory where the isotopic analysis will take place.

However, these two methods pose considerable problems.

With the first method, the filling of a pressurized bottle is anexpensive operation, time-consuming and not very practical to implementunder site conditions. Furthermore, H₂S is a gas which is toxic forhumans, and the transport, generally by air, of samples of compressedgas containing H₂S can be a very prolonged operation.

Finally, the quantity of H₂S present in the pressurized bottle may notbe sufficient to carry out the isotopic measurement.

The second method involves carrying out the oxidation-reduction reactionbetween the hydrogen sulphide and the cadmium acetate directly on site,and sending an aqueous solution containing a cadmium sulphideprecipitate to the analysis laboratory. Whilst this solution issatisfactory in terms of transport, it is difficult to implement. Infact, carrying out oxidation-reduction reactions on site requiresfragile laboratory equipment, not very compatible with the site, andpersonnel qualified to use it. Obtaining reliable and reproducibleanalysis results requires strict observance of the operating procedure,which is not always possible under site conditions.

Furthermore, for reasons of on-site safety, it is preferable to minimizehandling operations under site conditions.

Moreover, the application GB 2344365 describes a device suitable forsampling a determined quantity of fluid of hydrocarbons in situ in areservoir. A component of the device comprises a material capable ofreacting with hydrogen sulphide. The material can be for example ametal, a metal oxide, or an organic compound. However, the systemdescribed is mainly intended for measuring the concentration of hydrogensulphide directly in the fluid of the deposit. It is moreover extremelyheavy, complex and expensive to implement due to the difficulty oftaking a sample in situ.

Thus, there is a real need to develop a system allowing the isotopicmeasurement of the hydrogen sulphide which is robust, simple to useunder operating site conditions, safe and inexpensive.

SUMMARY OF THE INVENTION

The invention relates firstly to a method for sampling asulphur-containing solid product comprising:

-   -   supplying a gas flow comprising hydrogen sulphide;    -   bringing the gas flow into contact with a solid reagent and        reacting the solid reagent with the hydrogen sulphide contained        in the gas flow, said reaction fixing the sulphur of the        hydrogen sulphide by forming a sulphur-containing solid product        which is different in colour from the solid reagent; and    -   recovering the sulphur-containing solid product.

According to an embodiment, the solid reagent is constituted by cadmiumacetate crystals.

According to an embodiment, the gas flow originates from a deposit ofhydrocarbons.

According to an embodiment, the stage of bringing into contact iscarried out continuously and the gas flow is evacuated after having beenbrought into contact with the solid reagent.

According to an embodiment, the pressure in the contact chamber duringthe stage of bringing into contact is comprised between 0 and 10effective bar, preferably between 0 and 5 effective bar.

According to an embodiment, said method also comprises the followingstages:

-   -   transport of the sulphur-containing solid product to an        installation for the isotopic measurement of the sulphur;    -   isotopic measurement of the sulphur in the sulphur-containing        solid product within the installation for the isotopic        measurement of the sulphur.

A subject of the invention is also a device for sampling asulphur-containing solid product comprising:

-   -   a contact chamber, comprising a solid reagent, said solid        reagent being capable of reacting with hydrogen sulphide and the        reaction of the solid reagent with hydrogen sulphide producing a        change in colouration and fixing the sulphur of the hydrogen        sulphide by forming a sulphur-containing solid product;    -   gas flow delivery means, feeding the contact chamber inlet;    -   gas flow evacuation means, connected to the contact chamber        outlet; and    -   means of displaying the change in colouration of the solid        reagent in the detachable contact chamber.

According to an embodiment, the contact chamber is detachable.

According to an embodiment, the means of displaying the change incolouration of the solid reagent consist in the contact chamber having atranslucent or transparent wall, preferably made of plastic.

According to an embodiment, the contact chamber comprises two filters,upstream and downstream of the solid reagent.

According to an embodiment, the solid reagent is constituted by cadmiumacetate crystals.

According to an embodiment, said device comprises one or more of thefollowing components upstream of the contact chamber:

-   -   a pressure reducer;    -   a flow-control valve;    -   a purge system;

and optionally a flow meter downstream of the contact chamber.

According to an embodiment, the contact chamber is provided with meansfor breaking a wall of the contact chamber.

According to an embodiment, the method according to the invention isimplemented by means of the device according to the invention.

A subject of the invention is also a contact chamber comprising a solidreagent, said solid reagent being capable of reacting with hydrogensulphide and the reaction of the solid reagent with hydrogen sulphideproducing a change in colouration and fixing the sulphur of the hydrogensulphide by forming a sulphur-containing solid product, said contactchamber having a translucent or transparent wall, preferably made ofplastic.

The present invention makes it possible to overcome the drawbacks of thestate of the art. It provides more particularly a method for samplinghydrogen sulphide (and an associated device) allowing the isotopicmeasurement of the sulphur which is at the same time robust, easy to useunder production site conditions, safe and inexpensive.

This is achieved thanks to the use of a solid reagent capable ofreacting with the hydrogen sulphide and fixing the sulphur present inthe latter, by forming a sulphur-containing solid product, the reactionbeing accompanied by a change in colouration.

The invention offers more particularly at least one of the advantageouscharacteristic features listed below, or even two or more.

-   -   The use of a solid reagent makes it possible to avoid the        problems of transport and handling safety which are encountered        with liquid products or with compressed gas. In particular, the        sulphur-containing solid product is advantageously less toxic        and less corrosive than hydrogen sulphide,    -   The total period separating the sampling of a gas flow        comprising hydrogen sulphide and the isotopic measurement of the        sulphur is reduced. The frequency of the sampling and of the        isotopic measurement can thus be considerably increased.    -   The method according to the invention (and the associated        device) is easy to implement on a hydrocarbon production site,        without requiring the presence of a specialist technician.    -   Preferably, the device has no glass components, so that normal        safety measures are sufficient for its use.    -   The isotopic measurement of the sulphur allowed by the invention        is at least as reliable as an isotopic measurement carried out        by conventional methods.    -   The samples of sulphur-containing solid product can be kept for        a long time.    -   The change in colouration associated with the reaction makes it        possible on the one hand to verify the presence of hydrogen        sulphide, and on the other hand to adjust the period for which        the solid reagent is brought into contact with the gas flow        during the time strictly necessary for fixing a maximum quantity        of sulphur, without having to know beforehand the order of        magnitude of the hydrogen sulphide content of the gas flow.    -   The invention can be implemented at a low cost, in particular at        a cost hundreds of times lower than that of the method using        pressurized bottles.    -   The device according to the invention is miniaturized compared        with the existing systems, it takes up little space and can        easily be transported by hand in a case.    -   The sensitivity of the measurements is increased compared with        the method using pressurized bottles. In fact, it is sufficient        to circulate just as much gas flow as is necessary in the        contact chamber in order to obtain a sufficient quantity of        sulphur-containing material on which to carry out the isotopic        measurement.    -   The ease of use of the method makes it possible to carry out        numerous analyses, which makes it possible to increase the        sampling and in fine the reliability of the measurements.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic diagram of an embodiment of the deviceaccording to the invention.

FIG. 2 shows a detailed diagram of an embodiment of the contact chamberused in the device according to the invention.

FIG. 3 is a photograph of an example of a device according to theinvention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION p The invention is nowdescribed in more detail and in non-limitative manner in the followingdescription. Device for Sampling H₂S

With reference to FIG. 1 (and also in relation to the photograph of FIG.3), according to an embodiment, the sampling device according to theinvention comprises a gas flow delivery line 1. This can be for examplea stainless steel tube connected to a coil. This gas flow delivery line1 can be for example connected to a separator or to a wellheadinstallation, in the context of a use of the device on an oil or gasproduction site. It is also possible to connect the gas flow deliveryline 1 to a supply of inert gas (for example a nitrogen pressurizedbottle) or of compressed air in order to aerate the device in order toclean it, in particular between two sampling operations.

The gas flow delivery line 1 feeds a pressure reducer 2, intended toreduce the pressure of the gas flow. The pressure reducer 2 isadvantageously provided with a bleed line 3.

A first gas flow transport line 4, which feeds a flow-control valve 5,is connected to the outlet of the pressure reducer 2.

A second gas flow transport line 6, which supplies a two-way valve 7, isconnected to the outlet of the flow-control valve 5. This two-way valve7, according to its actuation, either purges the system via a bleed line8 (for cleaning the system for example by means of flushing with aninert gas), or supplies a contact chamber 10, via a third gas flowtransport line 9.

A fourth gas flow transport line 11, which supplies a flow meter 12,which can be in particular a ball flow meter and which makes it possibleto display whether the system is operating normally or whether it isblocked, is connected to the outlet of the contact chamber 10. Gasevacuation means 13 are connected to the outlet from the flow meter 12.

The gas evacuation means 13 can in particular evacuate the gas flowdirectly into the ambient atmosphere, taking account of the low gas flowrate which is in principle used within the framework of the invention,and taking account of the low hydrogen sulphide content of the gas flowat the outlet, since virtually all of the hydrogen sulphide contained inthe gas flow reacts in the contact chamber 10 when the device isoperating.

Advantageously, the whole of the device is capable of withstanding atemperature comprised between −30° C. and +50° C., preferably between−40° C. and +60° C., advantageously between −50° C. and +70° C. Thisproves important given the extreme temperature conditions which can beencountered in the various regions of the world where hydrocarbondeposits are situated.

The device preferably has no glass components. The device preferably hasno battery and no electric parts, in order that that the system does notpresent any detonation risk.

The whole of the device is preferably provided in an easilytransportable carrying case, for example made of stainless steel, (seeFIG. 3 in this connection). The maximum dimension of the carrying case(length) can be less than 1 m, preferably less than 80 cm or even lessthan 60 cm.

With reference to FIG. 2, the contact chamber 10 can be cylindrical inshape, preferably from 5 to 15 mm in external diameter and from 7 to 15cm in length, for example approximately 10 mm in external diameter and10 cm in length. The thickness of the wall 16 of the contact chamber 10can be approximately 2 to 5 mm, for example approximately 3 mm. Thecontact chamber can for example be produced using a Nalgene® tube.

At least part of the wall 16 must be transparent or translucent, so thatit is possible to observe the inside of the contact chamber 10. Forexample, it is possible to provide a window in the wall 16.Alternatively and preferably, the whole of the wall 16 is transparent ortranslucent. Advantageously, the wall 16 of the contact chamber is madeof plastic, for example polycarbonate or polyvinylidene fluoride(Kynar®). Preferably the plastic is unbreakable, i.e. shock-resistant.The wall 16 must be inert vis-à-vis hydrogen sulphide.

Inside the contact chamber 10 two filters 17 a, 17 b are arranged, whichform an obstacle to the passage of particles larger than 100 microns insize while still allowing the passage of the gases. The filters 17 a, 17b can for example be made of glass wool or cellulose, with optionally atubular component made of polypropylene to hold them in place.

The solid reagent 14 is arranged between the filters 17 a, 17 b.

Connectors 15 a and 15 b at both ends of the contact chamber 10 providethe connection respectively to the third gas flow transport line 9 andthe fourth gas flow transport line 11.

The solid reagent 14 is preferably constituted by crystal grains of areactive material. Alternatively, the solid reagent 14 can beconstituted by a support material (for example beads of silica oranother material) coated with reactive material.

The reactive material is chosen such that it is capable of reacting withhydrogen sulphide so as to provide a solid product while still fixingthe sulphur of the hydrogen sulphide, and so that the reaction with thehydrogen sulphide is accompanied by a change in colouration (i.e. thesolid product has a colouration different from that of the reactivematerial).

By change in colouration is meant a change in the colour of the materialwhich can be clearly distinguished by a user with the naked eye. Forexample, the change in colouration can be from white to yellow ororange.

Preferably, the reactive material is not a material the initial colourof which is dark, as this does not make it possible to correctly displaythe oxidation-reduction reaction with the hydrogen sulphide. The metalsand metal oxides are generally dark-coloured materials, the use of whichis not preferred. For example iron oxide is a black-coloured metal. Inthe presence of hydrogen sulphide, it rapidly changes to greenishferrous oxide then to rust-coloured ferric oxide, which is notconsidered to be a change in colour clearly distinguished by a user withthe naked eye.

The reactive material is therefore preferably a light-coloured (ideallywhite) solid material, for example cadmium acetate. Cadmium acetate, thecrystals of which are white, reacts with hydrogen sulphide to formcadmium sulphide (yellow crystals) and acetic acid.

The quantity of solid reagent in the contact chamber is preferablycomprised between 1 mg and 10 g, more particularly between 10 mg and 1g, in particular between 50 mg and 500 mg.

In the case where the solid reagent is constituted by cadmium acetatecrystals, a quantity of crystals of approximately 200 mg is appropriate.

In the device according to the invention, the contact chamber 10 ispreferably detachable, i.e. it can be removed from the device,preferably by hand, without the aid of tools, and can be replaced.

In this case, the contact chamber 10 can therefore be considered as adisposable item. Once removed from the device, the contact chamber 10can be transported, easily opened in order to release the solid productof the reaction and discarded.

As a result, the contact chamber 10 also constitutes a subject of theinvention in itself.

Alternatively, the contact chamber 10 can be permanently integrated intothe device. In this case, the contact chamber 10 is provided with anopening system, in order to be able to manually extract the solidproduct of the reaction.

Method for Sampling H₂S

The method according to the invention can be implemented using thedevice described above. In order to do this, the device is supplied witha gas flow containing hydrogen sulphide, in particular ahydrocarbon-based gas flow, for example originating from a productionwell.

The pressure reducer 2 makes it possible to reduce the pressure of thegas flow to a value of between 0 and 10 effective bar, preferablybetween 0 and 5 effective bar, for example approximately 2 effectivebar.

It is expedient to purge the device during start-up, by setting thetwo-way valve 7 to the purge position and opening the control valve 5.Then the control valve 5 is closed, the two-way valve 7 is set to thesampling position, and the control valve 5 is opened to a positionmaking it possible to achieve an appropriate flow rate.

The gas flow rate can be comprised for example between 10 and 100mL/min. A flow rate of 50 mL/min can be particularly suitable.

The gas flow passes successively, in a continuous flow, through thefirst gas flow transport line 4, the flow-control valve 5, the secondgas flow transport line 6, the two-way valve 7, the third gas flowtransport line 9, the contact chamber 10, the fourth gas flow transportline 11, the flow meter 12 and the gas evacuation means 13.

It is wise to keep the gas evacuation means 13 (like the bleed line 8)well away from the operators.

During the passage of the gas flow through the contact chamber 10, thehydrogen sulphide reacts with the solid reagent. Thus, the solid reagentis progressively converted to sulphur-containing solid product, whichleads to a change in colouration.

In the case where cadmium acetate crystals are used, the crystalsprogressively change colour from white to yellow. The change incolouration occurs first for the reagent which is situated on the sideof the third gas flow transport line 9 and then spreads to the reagentwhich is situated on the side of the fourth gas flow transport line 11.

According to a variant, once all of the solid reagent has changedcolour, the gas flow supply is cut.

According to another variant, the gas flow supply is cut when asubstantial part of the solid reagent has changed colouration (forexample at least 50%, or at least 60%, or at least 70%, or at least 80%,or at least 90%) but before all of the solid reagent has changedcolouration (i.e. before the change in colouration reaches the distalend of the tube). This variant is advantageous because in this case itis certain that H₂S will not leave the contact chamber, since all of itis consumed as the operation progresses, and is not delivered in excessinto the system.

If necessary, the method according to the invention comprises a stage ofestimating the change in colouration. For example, this estimation canbe carried out by comparing the colour of the material present in thecontact chamber (solid reagent or sulphur-containing solid product ormixture of these) with one or more standard colours.

Thus, the change in colouration makes it possible both to qualitativelyregister the presence of H₂S, but also to adjust the sampling of gas toexactly the necessary period, without having to precisely know the H₂Scontent of the gas, the flow rate in the device or the mass of solidreagent in the contact chamber 10.

In fact, the exact quantity of sulphur sampled by the method accordingto the invention is not critical from the point of view of an isotopicmeasurement of the sulphur. The method according to the invention is notprimarily intended to quantitatively measure the hydrogen sulphidecontent of the gas.

The period during which the gas flow is brought into contact with thesolid reagent can typically vary from one minute to several days orweeks depending on the H₂S content. A few minutes suffice in the case ofa flow of hydrocarbons with a high H₂S concentration, whereas a typicalperiod of 15 days is necessary for a gas with a low H₂S content (forexample approximately 2 ppm).

The change in colouration therefore makes it possible to specificallydetermine the period of exposure necessary in very varied cases. It isalso to be noted on this subject that the simplicity and the safety ofthe system are such that there is no need for an operator to be presentalongside the device for the whole of the sampling period. The operatoronly has to verify at what moment the change in colouration iscompleted, and then interrupt the circulation of the gas flow by closingthe control valve 5.

Once the circulation of the gas flow is interrupted, the operatorremoves the contact chamber 10 from the device. The contact chamber isplugged (for example by means of silicone plugs) and transported to aninstallation for the isotopic measurement of the sulphur, which willgenerally be a specialist laboratory equipped with a mass spectrometer.

The contact chamber is opened on site in a very simple manner, forexample by means of a knife or scalpel in the case where the wall 16 ofthe contact chamber 10 is made of plastic. Alternatively, it is alsopossible to provide breaking means on the contact chamber itself: forexample the contact chamber can be pre-slit so that it can be openedsimply by pressing with the fingers.

The sulphur-containing solid product is recovered, and an isotopicmeasurement of the sulphur is then carried out. The isotopic ratio ofthe sulphur in the sample and the isotopic deviation is thus measured,i.e. the deviation of this isotopic ratio compared with a standard(generally the isotopic ratio of the sulphates contained in seawater),according to the techniques well known to a person skilled in the art.This measurement provides information on the origin of the hydrogensulphide content in the initial gas flow, taking account of a set ofother parameters (temperature, age of the reservoir of hydrocarbons,type of alteration, composition etc.), as will be assessed by a personskilled in the art.

The method can then be implemented again for a new measurement using thesame device, after having cleaned the system by means of for exampleflushing with inert gas or compressed air (by operating the bleed lines3, 6), and having placed a new contact chamber in the location providedfor this purpose.

Alternatively, if the contact chamber is not detachable, thesulphur-containing solid product is recovered manually after opening thecontact chamber.

Then, in order to implement the method again, solid reagent is againinserted manually into the contact chamber, which is then closed again.

EXAMPLE

The following example illustrates the invention without limiting it.

In this example the measurement of isotopic deviation of sulphur 34(δ³⁴S) is compared by means of the standard method of bubbling through asolution of cadmium acetate and by means of the method described above(solid reagent: cadmium acetate crystals; mass of reagent: approximately100 mg). The actual isotopic measurements are carried out by means of amass spectrometer.

The results are given in Table 1 below.

TABLE 1 comparison of the measurement of isotopic deviation of sulphurby conventional sampling or according to the invention H₂S MeasurementMeasurement of content of δ³⁴S δ³⁴S according to Origin of the sample ofthe gas by bubbling the invention Standard gas 1%  +5.4‰ +5.3‰ (knownH₂S content) Gas from site No. 1 16%  +17.8‰ +19.4‰ (measured in 1995)Gas from site No. 2 7% −0.14‰ +2.0‰ Gas from site No. 3 2 ppm −30‰ onaverage −31.1‰ (measurements from 1992 to 2000)

It should be noted that the method according to the invention can beused over a very wide range of H₂S concentrations. Moreover theanalytical error produced is less than the natural variability of theisotopic deviation of sulphur.

1. A device for sampling a sulphur-containing solid product, comprising:a contact chamber, comprising a solid reagent, said solid reagent beingcapable of reacting with hydrogen sulphide and the reaction of the solidreagent with hydrogen sulphide producing a change in colouration andfixing the sulphur of the hydrogen sulphide by forming asulphur-containing solid product; gas flow delivery means, feeding thecontact chamber inlet; gas flow evacuation means, connected to thecontact chamber outlet; means of displaying the change in colouration ofthe solid reagent in the contact chamber; and means of recovering thesulphur-containing solid product from the contact chamber.
 2. The deviceaccording to claim 1, further comprising: means of transporting of thesulphur-containing solid product to an installation for the isotopicmeasurement of the sulphur; and means of isotopic measurement of sulphurof the sulphur-containing solid product within the installation for theisotopic measurement of sulphur.
 3. The device according to claim 1,wherein the contact chamber is detachable.
 4. The device according toclaim 1, wherein the contact chamber is cylindrical in shape, from 5 to15 mm in external diameter and from 7 to 15 cm in length.
 5. The deviceaccording to claim 4, wherein the thickness of the wall of the contactchamber is 2 to 5 mm.
 6. The device according to claim 1, wherein themeans of displaying the change in colouration of the solid reagentconsist in the contact chamber having a translucent or transparent wall.7. The device according to claim 6, wherein the wall of the contactchamber is made of plastic.
 8. The device according to claim 1, whereinthe contact chamber comprises two filters, upstream and downstream ofthe solid reagent.
 9. The device according to claim 1, wherein the solidreagent is constituted by cadmium acetate crystals.
 10. The deviceaccording to claim 1, further comprising one or more of the followingcomponents upstream of the contact chamber: a pressure reducer; aflow-control valve; a purge system.
 11. The device according to claim10, further comprising the following component upstream of the contactchamber: a flow meter downstream of the contact chamber.
 12. The deviceaccording to claim 1, wherein the device is capable of withstanding atemperature comprised between −30° C. and +50° C.
 13. The deviceaccording to claim 1, wherein the pressure in the contact chamber iscomprised between 0 and 10 effective bar.
 14. The device according toclaim 13, wherein the gas flow rate is comprised between 10 and 100mL/min.
 15. The device according to claim 13, wherein the gas flowpasses successively, in a continuous flow, through the first gas flowtransport line, the flow-control valve, the second gas flow transportline, the two-way valve, the third gas flow transport line, the contactchamber, the fourth gas flow transport line, the flow meter and the gasevacuation means.
 16. The device according to claim 1, wherein thedevice is in a transportable carrying case.
 17. The device according toclaim 16, wherein the transportable carrying case is made in stainlesssteel.
 18. The device according to claim 16, wherein the length of thecarrying case is less than 1 m.